The present invention relates to control systems for direct current electric motors and, more particularly, to a control system for providing smooth resistor staging of a chopper controlled electric traction motor.
Direct current (d-c) electric motors are often used in traction vehicle drive applications such as, for example, electric locomotives or transit cars. In such applications motive power is controlled by regulating motor current, typically by means of a control system employing a chopper. The chopper control system is essentially a controlled switching system connected in the energizing circuit of the motor armature so as to meter current to the motor by periodically opening and closing. The ratio of the closed time of the switching system to the sum of the closed time and the open time is the duty factor of the system. During the closed period of the chopper, the motor armature windings are connected to a power source through a path of relatively low resistance and current builds toward some peak value. During the open period of the chopper, the resistance in this energizing circuit is increased and armature current, circulating through a free wheeling diode, decays from the magnitude attained during the chopper closed time. In this manner, pulses of current are periodically applied to the motor and an average motor current is established. The average motor current tends to remain relatively constant due to the smoothing action of the circuit inductance. In general, the circuit inductance is sufficient to smooth the pulsating current and prevent jerking or lurching of the vehicle so long as the current pulses are supplied at relatively frequent rates, such as for example, 200 to 400 Hz.
An advantage of the chopper controlled d-c motor system is the relatively simply implementation of electrical braking. In electrical braking the d-c motor is operated as a generator with current generated by the armature windings being dissipated in a braking resistance (dynamic braking) or being forced back to the source (regenerative braking). The chopper control system operates in the braking mode in a manner similar to its operation in the driving mode, i.e., braking torque is regulated by the chopper by control of the average armature currents. In braking, however, the armature generated voltage may be allowed to be several times the magnitude of the source voltage in order to obtain the desired braking torque. Consequently, during the "off" time of the chopper, the peak voltage across the chopper may rise to a level several times as high as the source voltage. This braking characteristic necessitates the use of sophisticated and expensive components capable of withstanding such large applied voltages without destructive effect. One method which is commonly employed to avoid the necessity of using chopper components capable of withstanding such relatively high voltages is to connect the chopper in shunt with a first resistor which is serially interconnected with a plurality of additional resistors in the motor current path in a voltage dividing manner, thereby reducing the voltage impressed across the controlled switch in the chopper when turned off. In using this resistive voltage divider approach it is apparent that some means must be provided for removing the additional series connected resistors from the current path when it is desired to increase the average current in order to maintain the desired level of braking torque.
In order to remove the resistors from the motor current path it has been common practice to provide power switching means such as electromechanical contactors or cam actuated switches respectively associated with the additional resistors, thereby formaing a plurality of resistor stages. By actuating the power switching means, selected resistors may be short-circuited or by-passed. It will be appreciated that staging or short-circuiting of a resistor is only necessary when motor current must be increased to a magnitude greater than is possible by advancing the chopper duty factor to unity. It is also apparent that since each stage represents a finite resistance, short-circuiting of a stage will result in a step of voltage being applied to the motor. This step of voltage creates a current transient, which current transient may result in an unacceptable jerk or lurch of the vehicle. Although it would be possible to minimize this current transient by utilizing many small resistance stages rather than a few large stages, this approach would defeat the overall goal of economizing both cost and space.
In the operation of a control system using staging, the resistance stages are normally in series circuit arrangement with the chopper. When the duty factor of the chopper has been increased to its maximum limit and further voltage or current is required, a signal is directed to one of the resistance stages causing the associated contactor to close and shor-circuit the resistor thus raising the magnitude of voltage available for application to the motor. Obviously, unless some action is taken at the same time as the power switching means is actuated, a current transient may be applied to the motor resulting in a sudden jerk. The general practice is to size the resistors in each stage such that each is equivalent to the effective resistance represented by the difference in impedance of the chopper from maximum duty factor to minimum duty factor. The chopper can then be returned to its minimum duty factor when a stage is short-circuited and the net change in voltage applied to the motor will be nil. The chopper duty factor may then be smoothly increased to its maximum limit and the process repeated. The difficulty with such a method is in coordinating the actuation of the power switching means with the change of duty factor of the chopper. For example, at a 400 Hz pulse rate, an error or 1/2 second will allow 200 current pulses of an incorrect time duration to be applied to the motor and can result in a noticeable jerk. In practice of error of 0.040 seconds has been found to produce a noticeable jerk. Accordingly, various schemes have been devised for minimizing the effects of such stage changing.
One prior art approach to stage changing is illustrated in U.S. Pat. No. 3,581,172. In this patent a basic embodiment demonstrates stage changing by fully turning off the chopper at the same instant as a signal is sent to actuate a power switching means. The chopper is then maintained in a non-operative status until a fixed interval after the power switching means has closed, the interval being determined by the time required for the control circuit to change the chopper duty factor to a minimum limit. A further embodiment described in U.S. Pat. No. 3,581,172 employs current sensors associated with each resistor stage. In this embodimment the chopper remains at maximum duty factor until a power switch has been closed and a surge of current though the power switch has been detected. The chopper is then immediately returned to its minimum duty factor and held at that point for a time interval sufficient to allow the control system to recover and change to a minimum duty factor output command. The basic embodiment described above suffers from a relatively large gap in power applied to the motor during the time required for closing of the contactor. The closing time of commercially available contactors typically exceeds 0.1 second. Such a delay can result in an uncomfortable ride. In addition, the illustrated control system is placed in an open loop condition for more than 0.5 second during which time no corrections can be made for external influences such as, for example, a line voltage surge or a line voltage failure. The second embodiment overcomes the problem of power switch closing time but still places the control system in an open loop condition. Furthermore, the system does not respond until after a current surge has actually occurred thus raising a spectre of sudden lurch of the motor, the magnitude of which depends upon how fast the control system can be disabled. The method of stage chaning described in U.S. Pat. No. 3,581,172 thus falls short of ideal coordination of the staging transitions.
In addition to the staging problems during dynamic braking some d-c motor control systems rely on energy generated by the motor armature and stored in the line filter to control braking effort. These systems typically draw current from the line filter capacitor (which is connected across the input power terminals) and utilize this current to supply the motor field winding. Normally armature current is shunted through the free wheeling diode to the filter capacitor during chopper off time and maintains adequate stored energy to supply the field. However, in dynamic braking when the chopper is operating at maximum duty factor, the off time may be insufficient to maintain at at least a minimum level of energy in the filter capacitor and braking control may be lost. Accordingly, some means must be provided for assuring a minimum sustaining energy level in the line filter capacitor.
It is therefore an object of this invention to provide an improved d-c motor control system with smooth resistor staging.
It is a further object of this invention to provide an improved d-c motor control system including apparatus for maintaining at least a predetermined energy storage level for field control during dynamic braking.